In the manufacturing of semiconductors, many process steps require the measurement of various characteristics of films grown or deposited on the surface of a substrate or wafer. These characteristics include the film-thickness, the index of refraction, the reflectance, or even the concentration of various implanted atomic or ionic constituents. At times similar data about the substrate or wafer itself may be desired.
The conventional method for measuring films on a substrate, such as a semiconductor wafer, uses infrared light to illuminate one side of the wafer. The infrared light travels through the film and the wafer and is collected by a spectrometer on the other side of the wafer.
Another technique to obtain information related to the characteristics of the film or the substrate is to irradiate the sample with infrared light of one or more wavelengths, and collect the light that is reflected from the sample and any films on its surface. In reflection mode, the sample is supported by a planar surface, such as a wafer support or chuck. The chuck typically has a number of holes or channels, which are used to provide a vacuum for holding the sample on the chuck. In addition, by holding the sample tightly to the planar top surface of the chuck, the sample is held as flat as possible, which optimizes measurements.
In certain instances, a percentage of the incident light of a particular wavelength may penetrate through the films on the sample and through the sample itself. In such an instance, the flat areas of the chuck may specularly reflect a significant amount of the light that is transmitted through the sample and its films and is detected by the optical measurement instrument.
The optical measurement instrument includes a detector positioned above the sample that is used to collect the specularly-reflected light from the film and sample. Unfortunately, the detector also collects the specularly-reflected light from the sample supporting surface of the chuck. The light reflected from the chuck is detrimental to measurement, particularly where the sample supporting surface is not uniformly flat, but has miscellaneous holes and channels. Indeed, as reflected light measurements are made of the sample or its film, unknown quantitative contributions from the supporting chuck will reduce the accuracy or degrade the reliability of the desired measurements.